The invention relates to polyamide esters which can be prepared from tetracarboxylic dianhydrides and can be converted into polyimides by alcohol cleavage with cyclization, characterized in that the radicals OR in the ester groups --COOR, which are substituted during the polyimide formation, are alkoxy radicals having 2 to 5 C atoms which are monosubstituted or polysubstituted by fluorine.
Polyimides are known as orientation layers in liquid-crystal displays and have proven successful due to their good optical and thermal properties. They are also employed, in particular, in supertwist liquid-crystal display elements (SLC display elements). This is taken to mean relatively highly twisted display elements with a twist angle having a value in between 160.degree. and 360.degree., such as, for example, the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (German Offenlegungsschrift 3,503,259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Lennhouts, Appl. Phys. Lett. 50 (198U), 236), DST-LCDs (European Published Specification 0,246,842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
To produce supertwist display elements, orientation materials are required which induce a high tilt angle of the liquid crystal in the display; the angle should be at least about 3.degree..
It is known that polyimides prepared, for example, by reaction of the following components ##STR1## cause tilt angles of greater than 3.degree. (for example European Published Specification 231,781).
Polyimides of this type are usually prepared by first reacting tetracarboxylic acids with diamines to give polyamidocarboxylic acids, which are then cyclized with elimination of water to form polyimides (for example European Published Specification 0,249,881, European Published Specification 0,231,781 and European Published Specification 0,219,336). Alternatively, the tetracarboxylic anhydride can first be converted into another derivative, for example a dicarboxylic acid diester, after which the ester component is further reacted with an amine component to form the polyamidocarboxylic acid (U.S. Pat. No. 4,467,000).
European Published Specification 0,209,114 describes polyamide esters which can be converted into polyimides by alcohol cleavage. However, the invention relates to amphiphilic compounds, i.e. the alcohol component contains long alkyl chains.
Polyamide esters have furthermore been described in connection with photopolymerizable polyamide acid derivatives (for example European Published Specification 0,203,372).